The present invention relates to multifocal ophthalmic lenses. Such lenses are well known; they provide an optical power which varies continuously as a function of the position on the lens; typically when a multifocal lens is mounted in a frame, the power in the bottom of the lens is greater than the power in the top of the lens.
In practice, multifocal lenses often comprise an aspherical face, and a face which is spherical or toric, machined to match the lens to the wearer""s prescription. It is therefore usual to characterize a multifocal lens by the surface parameters of its aspherical surface, namely at every point a mean sphere S and a cylinder.
The mean sphere S is defined by the following formula:   S  =                    n        -        1            2        ⁢          (                        1                      R            1                          +                  1                      R            2                              )      
where R1 and R2 are the minimum and maximum radii of curvature, expressed in meters, and n is the refractive index of the lens material.
The cylinder is given, using the same conventions, by the formula:   C  =            (              n        -        1            )        ⁢          "LeftBracketingBar"                        1                      R            1                          -                  1                      R            2                              "RightBracketingBar"      
Such multifocal lenses are well known; among these multifocal lenses it is possible to distinguish lenses called progressive lenses, adapted for vision at all distances, lenses more specifically dedicated to near vision and to intermediate vision and lenses dedicated to far vision and to intermediate vision. Generally, the invention is applicable to any lens having a power variation.
Progressive ophthalmic lenses usually comprise a far vision region, a near vision region, an intermediate vision region and a main meridian of progression passing through these three regions. Document FR-A-2 699 294, to which reference may be made for further details, describes in its preamble the various elements of a progressive multifocal ophthalmic lens, together with work carried out by the applicant in order to improve the comfort for wearers of such lenses. In short, the upper part of the lens, which is used by the wearer for distance vision, is called the far vision region. The lower part of the lens is called the near vision region, and is used by the wearer for close work, for example for reading. The region lying between these two regions is called the intermediate vision region.
The difference in mean sphere between a reference point of the near vision region and a reference point of the far vision region is thus called addition. These two reference points are usually chosen on the main meridian of progression defined below.
For all multifocal lenses, the power in the various far, intermediate and near vision regions, independently of their position on the lens, is determined by the prescription. The latter may comprise just a power value for near vision or a power value for far vision and an addition, and possibly an astigmatism value with its axis and prism.
Lenses dedicated more specifically to near vision do not really have a far vision region as with conventional progressive lenses, but a near vision region and a lower power region above this near vision region; the near vision region provides the wearer with clear and comfortable vision in near vision, that is to say in a plane located at about 30 cm; the decrease in power beyond this distance, in the upper part of the lens, allows the wearer to see clearly beyond that. These lenses are prescribed depending on the power needed by the wearer for near vision, independently of the far vision power.
Such a lens is described in an article of the Opticien Lunetier of April 1988, and is marketed by the applicant under the Essilor Delta brand; this lens is simple to use and as easy to put up with as a progressive lens, and is attractive for the presbyopic population not equipped with progressive lenses. This lens is also described in patent application FR-A-2 588 973. It has a central part which is equivalent to the unifocal lens that would normally be used to correct the presbyopia, so as to provide satisfactory near vision; this central part corresponds substantially to a near vision region of a progressive multifocal lens. In addition, it has a slight power decrease in the upper part, which also provides the wearer with clear vision beyond the normal field of near vision.
For progressive lenses, a line which is representative of the intersection of the aspherical surface of a lens with the gaze of an average wearer when he or she looks straight ahead at objects in a meridian plane, at different distances, is called the main meridian of progression. On the multifocal surface, the main meridian of progression is often an umbilical line, in other words, one for which all points have zero cylinder. This line is used in the definition of a progressive surface, as an optimization parameter. It is representative of the strategy for using the lens by the average wearer. Numerous choices for the meridian have been proposed; the simplest and the oldest consists in making a vertical umbilical line on the lens, and in inclining each lens on mounting in a frame, in order to take into account convergence of the gaze on passing from near vision to far vision.
On the aspherical face of a multifocal lens, this definition of the reference meridian corresponds substantially to a line which is formed from the middles of the horizontal segments connecting the half-addition isocylinder lines. In this context, the set of points having a given cylinder value is called an isocylinder line for this cylinder value.
A point, called a mounting center, is commonly marked on ophthalmic lenses, whether they are progressive or not, which point is used by the optician for mounting lenses in a frame. From the anthropometric characteristics of the wearerxe2x80x94pupil separation and height with respect to the framexe2x80x94the optician machines the lens by trimming the edges, using the mounting center as a reference point. In lenses marketed by the applicant, the mounting center is located 4 mm above the geometric center of the lens; the center is generally located in the middle of the micro-etchings. For a lens correctly positioned in a frame, it corresponds to a horizontal direction of viewing, for a wearer holding his/her head upright.
The applicant has also proposed, in order to better satisfy the visual requirements of presbyopic persons and to improve the comfort of progressive multifocal lenses, adapting the shape of the main meridian of progression according to the power addition, see patent applications FR-A-2 683 642 and FR-A-2 683 643. FR-A-2 753 805 proposes plotting the meridian by ray tracing and allows the meridian to be determined, while taking account of bringing the reading plane closer and of the prismatic effects.
One of the problems which arises is that of mounting multifocal lenses in small frames; it happens during mounting of such lenses in small frames, that the lower part of the near vision region is removed on machining the lens. The wearer then has correct vision in far vision and in intermediate vision, but a near vision region which is too small in size. He or she tends to use the lower part of the intermediate vision region for near vision. This new problem is particularly acute because of the fashionable trend for small frames.
Another problem encountered by wearers of progressive multifocal lenses is tiredness in the case of extended work in near vision or in intermediate vision. This is because the near vision region of a progressive lens is in the bottom of the lens, and extended use of the near vision region may cause tiredness with some wearers.
A final problem is adaptation by the wearers to the lenses. It is known that wearers commonly have need of a period of adaptation to progressive lenses, before using the various lens regions in a suitable manner for the corresponding activities. The adaptation problem is encountered in particular for former bifocal lens wearers; these lenses have an area for near vision, the upper part of which is generally located 5 mm under the geometric center of the lens. However, in conventional progressive lenses, the near vision region is generally located lower; even if it is difficult to exactly determine the limit between the intermediate vision region and the near vision region, a wearer would be subject to less tiredness by using progressive lenses with near vision at 5 mm below the mounting center.
The invention provides a solution to these problems. It provides a lens capable of being mounted in small frames, without the near vision region being reduced. It also improves the comfort of wearers on extended use of the near vision region or the intermediate vision region. It also makes it easier for former wearers of bifocal lenses to adapt to the progressive lenses. More generally, the invention is applicable to any lens having a fast power variation; it provides a particularly advantageous compromise between the power variation and the maximum cylinder value.
More specifically, the invention provides a multifocal ophthalmic lens, comprising an aspherical surface with a mean sphere and a cylinder at every point thereof, characterized by the equation:
L less than 1/(xe2x88x920.031xc3x97R2+0.139xc3x97R+0.014) 
where L is equal to the ratio (Smaxxe2x88x92Smin)/gradSmax, the ratio of the difference between the maximum and minimum values of the mean sphere in a region of a 40 mm diameter circle centered on the geometric center of the lens, this region being limited by vertical straight lines at 1 mm from this center on the temporal side and at 4 mm from this center on the nasal side, on the one hand, to the maximum value of the gradient of the mean sphere in this same region, on the other hand, and
where R is equal to the ratio Cmax/(Smaxxe2x88x92Smin), the ratio of the maximum cylinder value inside said circle to the difference between the maximum and minimum values of the mean sphere in said region.
In one embodiment, the lens has a near vision region.
It may also have a far vision region.
In one embodiment, the angle between two half-lines coming from the geometric center of the lens and passing respectively through the points of a 20 mm radius circle centered on the geometric center which have a cylinder equal to half the difference (Smaxxe2x88x92Smin) between the maximum and minimum values of the mean sphere in said region and which are located in the upper half of the lens is between 130xc2x0 and 155xc2x0.
It is further advantageous that the angle between two half-lines coming from the geometric center of the lens and passing respectively through the points of a 20 mm radius circle centered on the geometric center which have a cylinder equal to half the difference (Smaxxe2x88x92Smin) between the maximum and minimum values of the mean sphere in said region and which are located in the lower half of the lens is between 40xc2x0 and 55xc2x0.
In one embodiment, at a point located on a half-circle of 20 mm radius centered on the geometric center in the upper part of the lens, and which has a cylinder equal to half the difference (Smaxxe2x88x92Smin) between the maximum and minimum values of the mean sphere in said region, the ratio of the cylinder gradient to said difference is between 0.03 and 0.11 mmxe2x88x921.
In another embodiment, at a point located on a half-circle of 20 mm radius centered on the geometric center in the lower part of the lens, and which has a cylinder equal to half the difference (Smaxxe2x88x92Smin) between the maximum and minimum values of the mean sphere in said region, the ratio of the cylinder gradient to said difference is between 0.05 and 0.014 mmxe2x88x921.
It is also possible that the ratio of
the maximum cylinder gradient on the two points located on a half-circle of 20 mm radius centered on the geometric center in the lower part of the lens, and which have a cylinder equal to half the difference (Smaxxe2x88x92Smin) between the maximum and minimum values of the mean sphere in said region, on the one hand, to
the minimum cylinder gradient on the two points located on a half-circle of 20 mm radius centered on the geometric center in the upper part of the lens, and which have a cylinder equal to half the difference (Smaxxe2x88x92Smin) between the maximum and minimum values of the mean sphere in said region, on the other hand, is less than 2.
Preferably, for a point located on a 20 mm radius circle centered on the geometric center, and whose mean sphere is greater than the minimum mean sphere Smin by an amount equal to half the difference (Smaxxe2x88x92Smin) between the maximum and minimum values of the mean sphere in said region, the ratio of the sphere gradient to said difference is between 0.015 and 0.07 mmxe2x88x921.
In yet another embodiment, the angle from which two points located on a half-circle of 20 mm radius centered on the geometric center of the lens in the upper part of the lens and which have a cylinder equal to half the difference (Smaxxe2x88x92Smin) between the maximum and minimum values of the mean sphere in said region are seen from said center is at least equal to twice the angle from which two points located on a half-circle of 20 mm radius centered on the geometric center of the lens in the lower part of the lens and which have a cylinder equal to half said difference (Smaxxe2x88x92Smin) are seen from said center.